Yes, unfortunately I was in that case where pagination simply wasn't provided by the feed. Push notifications of course wouldn't have worked (if they were even supported, and I don't believe they were), since otherwise I would have just sat there polling the feed itself every 20 minutes like a maniac.
It seems like this widespread limitation leads to the reliance on aggregators, and second-hand it seems google playing the disappearing act with their aggregation service was one of contributing factors to (or a symptom of?) the decline of RSS. Certainly I would have been willing to just shrug my shoulders and use it if it was still operating when I ran into my one and only case of wanting to use an RSS feed.
However WebSub requires that you have an "always online" device to receive the notification and very few feeds and readers support pagnation.